The present invention concerns a fluid catalytic process wherein (a) residuum and other heavy oils containing sulfur compounds are cracked to produce useful products, (b) sulfur-containing coke on the used catalyst is gasified using a steam-air mixture at a temperature from about 1100.degree. F. to about 2200.degree. F. to produce a low BTU gas and hydrogen sulfide, (c) the partially decoked catalyst is regenerated by full combustion of the remaining coke on the used catalyst wherein the flue gas contains sulfur oxides, and (d) the regenerated catalyst is returned to the heavy oil cracker for re-use.
The treatment of a reduced crude or residual oil to produce hydrocarbon products of greater economic value than petroleum coke has long been a problem in the petroleum industry. The nature of the feed, including sulfur content, and the need to improve production of liquid products including gasoline from all available petroleum sources are aspects of the problem. Catalytic cracking of the residual oil causes deposits of coke upon the catalyst so catalyst material is continuously withdrawn from the cracking unit and sent to a regenerator where the coke is burned off. High coke yields from cracking residual oils require removal of large quantities of excess energy as heat from the regenerator and reduce production of liquid products. Moreover, although catalytic cracking of residual oils can be more attractive economically than other processes for utilizing the residual oils, the required regenerator can be an extremely large economic investment because of the necessity of auxiliary means for removing the excess heat generated by the combustion of the coke in excess of the reactor requirements. An associated problem is the high sulfur content of the coke which results in the formation of environmentally unacceptable amounts of sulfur oxides during combustion.
The catalytic cracking of various heavier mineral hydrocarbons, for instance, petroleum or other mineral oil distillates such as straight run and cracked gas oils; petroleum residues, etc., has been practiced for many years. The term "gas oil" is a broad, general term that covers a variety of stocks. The term includes a light gas oil (boiling range 400.degree. to 600.degree. F.), heavy gas oil (boiling range 600.degree. to 800.degree. F.) and vacuum gas oils (boiling range 800.degree. to 1100.degree. F.) The petroleum residues have a boiling range from about 1100.degree. F. and up. The vacuum gas oils and residuals together represent the atmospheric reduced crude.
A residual stock is in general any petroleum fraction with a higher boiling range than gas oils. Any fraction, regardless of its initial boiling point, which includes the heavy bottoms, such as tars, asphalts, or other undistilled materials can be termed a residual fraction. Accordingly, a residual stock can be the portion of the crude remaining undistilled at about 1050.degree.-1200.degree. F., or it can be made up of a vacuum gas oil fraction plus the portion undistilled at about 1050.degree.-1200.degree. F. For instance, a topped crude may be the entire portion of the crude remaining after the light ends (the portion boiling up to about 400.degree. F.) have been removed by distillation. Therefore, such a fraction includes the entire gas oil fraction (400.degree. F. to 1050.degree.-1200.degree. F.) and the undistilled portion of the crude petroleum boiling above 1050.degree.-1200.degree. F.
A great deal of effort has been applied in petroleum refining to increase recovery of catalytic cracking feedstock or "gas oils" from residual fractions of petroleum oil, but attempts to employ heavier fractions of crude oil for catalytic cracking have been limited due to the heavy coke laydowns experienced in cracking such feedstocks. Coke build-up in catalytic cracking is caused by a number of factors not necessarily independent of each other. The presence of high-boiling aromatics and other hydrocarbon coke-formers in the feed and lack of selectivity in the catalyst contribute greatly to excess coke formation. In high boiling feedstocks, both of these problems are more severe since these fractions contain higher proportions of coke formers than conventional gas-oil feedstocks. Combustion of the coke formed results, as mentioned, in generation of heat in excess of reactor requirements.
Petroleum fractions containing large amounts of coke-forming components such as the asphaltic and residual materials described above can be hydrotreated to reduce coke formation. However, high boiling fractions frequently can require such severe hydrotreating to give the hydrocarbon an improved hydrogen-to-carbon ratio to make them trouble-free cracking feeds with concurrent control of coke formation and excess heat that the expense of hydrotreating is not practical. Other economically-expensive solutions to the problem of coke-formation have been proposed in the prior art.
Sulfur is also typically present in a reduced crude or residual oil. During the cracking process, some of this sulfur is deposited in the coke which is produced by the cracking process. During the conventional regeneration process sulfur oxides are produced during oxidation of the coke to carbon dioxide.
In the residual oil cracking process, the catalyst material is typically withdrawn continuously from the cracking unit and sent to a regenerator where the coke is burned off. High coke yields from cracking residual oils requires removal of a large quantity of excess energy as heat from the regenerator. When the coke is burned in the regenerator, the sulfur content of the coke is converted to sulfur oxides which are emitted in the flue gas and this may necessitate stack gas scrubbing or some other means of control. Moreover, although catalytic cracking of residual oils can be more attractive than other processes for utilizing the residual oils, an extremely large economic investment can be required because of the necessity of auxiliary means of removing the excess heat generated by the combustion of the coke in excess of the reactor requirements. An accompanying problem is the economic investment required for regenerator stack gas scrubbing. When this coke is burned in the regenerator of a catalytic cracker, this sulfur is converted to sulfur oxides.
In the prior art relative to cracking residua, Brown, et al., U.S. Pat. No. 2,885,350, teaches cracking of a heavy hydrocarbon such as residuum in the presence of activated coke and hydrogen under pressure wherein the production of the required activated coke and hydrogen are obtained by reacting steam with coke in a separate reactor. Brown, U.S. Pat. No. 2,885,350 also teaches that surplus hydrogen in the tail gas may be used for further hydrogenation or desulfurization or other after-treatment of the product withdrawn from the process. Riblett, U.S. Pat. No. 2,606,430, teaches high-temperature carbonization and gasification of coke produced by cracking to yield synthesis gas. The temperature in the cracking zone is within the range of about 900.degree. to 1,200.degree. F., about 1,400.degree. to 1,600.degree. F. in the carbonization zone, and about 2,000.degree. F. in the gasification zone, heat being supplied to the process by the reaction of the coke. Excess coke product is recycled. Watkins, U.S. Pat. No. 3,017,250, teaches a process for the production of hydrogen wherein iron-containing particles are reacted with steam to produce hydrogen, at a temperature of about 1,100.degree. to 1,300.degree. F., the iron particles thereupon being passed to a reactor to thermally crack a hydrocarbon oil at a temperature of from 900.degree. to 1,100.degree. F. The resulting coke-laden iron particles are regenerated at a temperature of from about 1,500.degree. to 1,600.degree. F., the heat for process being supplied by the hydrocarbon oil and combustion of the reducing gases therefrom. Paterson, U.S. Pat. No. 3,172,840, teaches the coking of residuum boiling above 750.degree. F. to produce coke and a liquid distillate, the distillate being hydrocracked to light ends and gasoline, the coking zone being a delayed coker, and the coke removed from the coke drums as product. Hamner, et al., U.S. Pat. No. 3,179,584, teaches a process for increasing hydrogen production in coking of residual oils by addition of an aqueous caustic solution to the residual hydrocarbon oils. The solution and oil feed are emulsified prior to coking, heated to a temperature between 150.degree. and 350.degree. F., and coked or cracked between 850.degree. and 1,250.degree. F. at between 0 and 200 psig. Johnson, et al., U.S. Pat. No. 3,542,532, teaches a process for production of a gas rich in hydrogen from petroleum coke having a particular size range wherein the coke is gasified with steam. The product of the process is a hydrogen and carbon monoxide containing gas. A catalyst is not used. Temperatures in the transfer-line burner range from 1,800.degree. to 2,400.degree. F. and in the reactor wherein the coke reacts with the steam to produce hydrogen from 1,200.degree. to 2,400.degree. F. Kimberlin, et al., U.S. Pat. No. 3,726,791, teaches that high Conradson carbon feeds are coked to lay down extensive carbon deposit on a gasification catalyst. The coked catalyst is then steam gasified to produce hydrogen. The catalyst is a Group V-B, VII-B, or VIII metal oxide on a support of gamma alumina, bauxite, or activated clay. Lawson, U.S. Pat. No. 3,008,896, teaches the catalytic cracking of residual oils under conditions yielding only about 30 percent conversion of the residual oil to provide gas oils for later gas oil catalytic cracking wherein oil is occluded or absorbed on the catayst which is also later cracked at a higher temperature. Leaman, et al., U.S. Pat. No. 3,433,732, teaches catalytic hydrocracking and steam regeneration of the catalyst to produce hydrogen employing a catalyst containing crystalline aluminosilicate. Thomas, et al., Canadian Pat. No. 875,528 teaches a process for production of hydrogen wherein a coked catalyst is reacted with oxygen and carbon dioxide to produce carbon monoxide, the carbon monoxide thereupon being reacted with steam over a catalyst to form hydrogen and carbon dioxide.
As an economic alternative to the solutions in the prior art, this invention employs a stripper and a gasifier to control production of excess heat and heavy coke laydown by partially removing coke deposits on the spent catalyst and producing fuel gas. The partially regenerated catalyst from the gasifier with partially-removed coke deposits undergoes complete carbon removal in the regenerator.
The primary object of this invention is to control the production of excess heat in the regeneration of catalysts caused by the production of excessive amounts of coke in the catalytic cracking of residual oils by providing an integrated process for the treatment of residual oils in which the steps of catalytic cracking of the residual oil, the regeneration of the coked catalyst, and the removal of sulfur compounds are combined and adjusted to maximize production of liquid products.
Another object of this invention is to provide an improved method for removing sulfur compounds from heavier petroleum fractions such as heavy fuel oils, residuum, etc.
Another object of this invention is to provide a process for catalytic cracking of residual oil with flexibility to handle differences in feedstocks or in the required conversion to cracked products. A further object of this invention is to provide a process for catalytic cracking of residual oil wherein control of sulfur emissions is obtained. A further object of this invention is to provide a regenerated catalyst. Another object of this invention is to provide a method for regenerating a coked catalyst wherein the coke is gasified off with steam and air or oxygen. Another object is to provide a regeneration method wherein the flow of coked catalyst to the regeneration cycle is controlled and, thereby, the regenerator temperature. These and other objects will become clear from the following specification.
Accordingly, it is generally known to convert heavy petroleum hydrocarbons in the presence of a catalyst and/or hydrogen or by thermal cracking to produce coke and low BTU gases. It is also well-known to regenerate a coked catalyst with steam to produce hydrogen or with oxygen and carbon dioxide to produce carbon monoxide. However, a process with control of excess heat produced by combustion of coke and with flexibility to handle residual oils with differing characteristics has not been known in the prior art wherein a catalyst in a transfer-line reactor is used in a fluidized bed to crack reduced crude to gasoline and other valuable liquid and gaseous products and to coke, and wherein the coke deposited on the catalyst is gasified with oxygen and steam in a separate stripper with control of excess heat to produce a low BTU fuel gas comprising sulfur compounds if present, hydrogen, methane, carbon monoxide, and carbon dioxide, and the catalyst is further regenerated in a regenerator to produce a regenerated catalyst.